Electrophotography is in extensive use in copiers, printers, and printing machines because of advantages thereof including the ability to instantaneously give high-quality images. With respect to the electrophotographic photoreceptor (hereinafter sometimes referred to as “photoreceptor”), which is the nucleus of electrophotography, photoreceptors employing organic photoconductive substances are in extensive use because these photoconductive substances have advantages such as freedom from pollution, ease of film formation, and ease of production.
Image forming apparatus based on electrophotography are being required to attain higher image quality, higher speed, and higher durability year after year. Although processes conducted on the periphery of the photoreceptor, such as charging, exposure, development, and transfer, also are being individually improved in order to meet those requirements, the improvements are not always sufficient or, in many cases, are not adopted for reasons of cost. In such cases, improvements in receptors are necessary, but there is little room for improvement by photoreceptor design.
For example, in the case of using a toner which has a shape close to sphere, such as a chemical toner, cleaning is difficult and, hence, an often employed technique is to heighten the pressure for touching the cleaning blade to the photoreceptor. In this case, not only the degree of wear of the photoreceptor increases, but also problems are prone to arise, such as adhesion of a component of the toner to the photoreceptor surface (filming), the occurrence of scratches, and the chatter of the cleaning blade (noise). There are often cases where such problems are desired to be solved by improving not the development system or cleaning system but the composition of the photoreceptor. Meanwhile, if the problems can be mitigated by an improvement in photoreceptor composition, the development systems and cleaning systems according to conventional techniques can be used as such and, hence, this solution is advantageous also from the standpoint of cost.
With respect to compositional improvements in photoreceptors also, gas resistance and surface properties including surface hardness are being improved by various methods (patent documents 2 to 6). However, there are various limitations. For example, in the case where the electrical responsiveness of a photoreceptor is desired to be enhanced in order to meet the request for an increase in speed, a usual technique is to increase the proportion of the charge transport substance in the photosensitive layer to the binder resin. However, the resultant photosensitive layer is prone to wear and is unable to meet the request for higher durability. Meanwhile, replacement of the binder resin, which is a conventional polycarbonate resin, with a polyester resin having high durability makes it impossible to meet the request for higher responsiveness, because the polyester resin is inferior in electrical property. Such combinations of inconsistent performances are often encountered when photoreceptor compositions are designed. Consequently, a key to development is how to conquer the problem and reconcile the required performances.
Under these circumstances, patent document 1 has proposed a charge transport substance having a large conjugated system. This charge transport substance exhibits a high charge mobility even when used in a small amount relative to the binder resin amount, and shows an exceedingly low residual potential. This charge transport substance hence has the possibility of reconciling electrical properties and wear resistance.